The present inventor's prior U.S. patent application, Ser. No. 07/503,313 filed on Mar. 30, 1990, and entitled "Motion-Adaptive Video Noise Reduction System Using Recirculation and Coring" represented a considerable and successful effort to realize a very efficient luminance/chrominance noise reducer employing frame rate recirculation and resultant noise reduction. While the unit described in the referenced patent application works particularly well as a stand-alone unit, presently the costs of the digital components necessary to implement the principles described therein are sufficiently high as to make it impractical to include those concepts and embodiments within a digital television set intended for the mass consumer marketplace.
With the advent of application-specific digital large scale integrated circuits, it is practical to realize and achieve a truly digital television set at a sufficiently low price as to enable the set to be marketed to consumers for home use. In such a set, while the input signal is analog NTSC for example, the output is line doubled red, green and blue display driving signals (as well as the necessary synchronization and driving signals for the display).
Temporal domain comb filter separators are known for separating chrominance and luminance components of a quadrature modulated subcarrier color television signal. Such comb filter structures typically employ a single frame delay, or more preferably employ two single frame delays connected in tandem. Fractional gain amplifiers and a summing junction enable the function -1/4V1+1/2V2-1/4V3 to be combined so as to separate chrominance from a baseband signal on a frame by frame basis. The advantage of a temporal domain (frame delay based) comb filter separator over a spatial domain (line delay based) is that resolution along diagonal dimensions, particularly edges along .+-.45 degrees, is not degraded for stationary picture images. The disadvantage of a temporal domain comb filter separator is that it ceases to function effectively during conditions of motion in the picture. Adaptivity techniques, such as motion adaptivity, and/or bidimensional comb filtering techniques, may be employed to ameliorate somewhat the disadvantage of the breakdown of temporal domain comb filter separation in the presence of motion in the picture. The present applicant's published European Patent Application, No. 0 218 241 published on Apr. 15, 1987, illustrates salient aspects of a bidimensional comb filter decoder, particularly in conjunction with FIG. 5 thereof.
While the cost of digital memory integrated circuits is constantly being reduced, and digital video frame stores are readily implemented, even within consumer television set comb filter decoders, the subjective improvement along diagonal transitions in the color picture remains offset by the still not insubstantial costs of such memory circuits. Thus, one hitherto unsolved need has remained for providing additional functionality to the frame delays included within temporal domain comb filter separators at the receiver.
While it is conceivable that the noise reduction embodiments given in the referenced parent application can be modified to make use of the frame delays included within the temporal domain comb filter separator, the modification would necessarily be cumbersome and difficult to realize in practice.